Recently, a demand for introduction of a next generation 40 Gbit/s optical transmission system has been increased, and in addition to this, a transmission distance and a frequency use efficiency equivalent to a 10 Gbit/s system are desired. As a means for satisfying the above demand and desire, researches and developments of the DPSK (Differential Phase Shift Keying) modulation scheme have been actively performed, since the DPSK modulation scheme is superior to the NRZ (Non Return to Zero) modulation scheme having been previously applied to systems of 10 Gb/s in optical signal-to-noise ratio (OSNR) tolerance and non-linearity tolerance. Further, in addition to the above-mentioned modulation scheme, researches and developments of other phase modulation schemes such as DQPSK (Differential Quadrature Phase-Shift Keying) modulation featured in its narrow spectrum (high-frequency use efficiency) have also been actively performed.
In particular, the DQPSK modulation scheme transmits two phase-modulated digital signals which are at the same time using signal light of one frequency. Since this scheme needs a pulse repetition frequency only a half (for example, 20 GHz) of the data speed (for example, 40 Gbit/s) of transmission data, a signal spectrum width becomes half of that of the previous NRZ modulation scheme or the like, so that the DQPSK modulation scheme is superior to the previous NRZ modulation scheme or the like in a frequency use efficiency, wave dispersion tolerance, device transmission characteristics, and so on. For the above reason, in the field of optical transmission systems, application of the present modulation scheme to high-speed transmission systems, in particular, systems in which the data speed exceeds 40 Gbit/s, has been studied actively.
FIG. 17 is a diagram illustrating a common type of DQPSK modulator 100. A technique for transmitting data through modulation/demodulation in the DQPSK scheme is also described in, for example, the following patent document 1.
The DQPSK modulator 100 shown in FIG. 17 is provided for, for example, an optical transmitter that transmits an optical signal in an optical transmission system to modulate a data signal into an optical signal with the DQPSK modulation scheme, and includes not only a transmission data processor 101, amplifiers 102-1 and 102-2, a CW (Continuous Wave) light source 103, a π/2 phase shifter 104, and two Mach-Zehnder phase modulators 105-1 and 105-2, but also an MZM interferometer 106 that makes phase modulation signals from the phase modulator 105-1 and 105-2 interfere, which phase modulation signals are given a phase difference of π/2.
That is, the MZM interferometer 106 is connected thereto with a CW light source 103 on the input end, and there formed are phase modulators 105-1 and 105-2 at parts of bifurcated waveguides, respectively. Hereinafter, a Mach-Zehnder waveguide operating as the MZM interferometer 106 will sometimes be referred to as a parent MZ (Mach-Zehnder) waveguide, and a Mach-Zehnder waveguide operating as a phase modulator formed at a bifurcated waveguide part of the parent waveguide will sometimes be referred to as a child MZ waveguide.
Here, the transmission data processor 101 has a function as a framer and an FEC encoder together with a function as a DQPSK precoder that performs coding processing in which difference information between the current code and the code previous thereto by one bit is reflected. A data signal output from this transmission data processor 101 is output as signals obtained by dividing coded data of, for example, approximately 40 Gbit/s into two series of coded data (data #1 and data #2) of 20 Gbit/s. Further, the amplifiers 102-1 and 102-2 each amplify the coded data (data #1 and data #2, respectively) and outputs the amplified data to the phase modulators 105-1 and 105-2 as driving signals.
Further, the CW light source 103 outputs continuous light. The continuous light output from the CW light source 103 is bifurcated by the split waveguide 106a forming the MZM interferometer 106. The I (In-phase) arm, one of the bifurcated waveguides, is input to the phase modulator 105-1; the Q (Quadrature-phase) arm, the other of the waveguides, is input to the phase modulator 105-2. Each of the phase modulators 105-1 and 105-2 has a structure basically similar to that of a common BPSK (Binary phase-shift Keying) modulator.
Here, the phase modulator 105-1 modulates continuous light from the CW light source 103 with one of the series of coded data (data #1) from the transmission data processor 101 and outputs an optical signal which carries information on a binary light phase (0 rad or π rad). Further, the phase modulator 105-2 modulates continuous light from the CW light source 103 with the other of the series of coded data (data #2) from the transmission data processor 101. Then, the π/2 phase shifter 104 phase-shifts the thus modulated optical signal by φ=π/2, thereby outputting the phase-shifted light as an optical signal which carries information on a binary light phase (π/2 rad or 3π/2 rad).
Then, the modulated light from the phase modulators 105-1 and 105-2 are combined by the combining waveguide 106b forming the MZM interferometer 106 and then output. That is, combination of modulated light from the phase modulators 105-1 and 105-2 makes it possible to output an optical signal whose intensity is fixed but which carries information on a quadrature (π/4, 3π/4, 5π/4, and 7π/4) light phase, that is, a DQPSK modulated optical signal.
In this manner, in DQPSK modulation, light transmission is performed with a 4-ary symbol of π/4(0,0), 3π/4 (1,0), 5π/4 (1,1), and 7π/4 (0,1) in which data “0” and “1” are modulated into phase 0 and phase π, respectively, by means of shifting two series of digital signals by π/2, thereby making the two series of digital signals interfere.
Here, the π/2 phase shifter 104 also performs phase shifting of input signal light with a voltage applied through an electrode, and a voltage (bias voltage) applied in the π/2 phase shifter 104 and the phase shift amount of the optical signal has a one-to-one correspondence relationship as indicated by A in FIG. 18. Thus, the π/2 phase shifter 104 shown in FIG. 17 supplies a fixed voltage V1 with which an optical signal from the phase modulator 105-2 is distorted by π/2.
However, the DQPSK modulator 100 described above with reference to FIG. 17 has a problem of an occurrence of a deviation of α from π/2 that is an ideal phase difference between a phase modulation signal component (see B in FIG. 19) from the I arm side and a phase modulation signal component (see C in FIG. 19) from the Q arm side, in the phase modulated optical signal obtained from CW light (see A in FIG. 19) due to production tolerance of the MZM interferometer 106, change in ambient temperature, and other temporal factors (see D in FIG. 19).
In other words, since a correspondence relationship A shown in FIG. 18 changes as indicated by A′ due to the above described factors, it becomes impossible to make the optical signals from the phase modulators 105-1 and 105-2 have a predetermined phase difference therebetween at the time they pass through the combining waveguide 106b only by means of giving a fixed voltage V1.
As a result, a phase difference of π/2 is not present between 4-ary symbols, and a peak power between symbols changes (see D in FIG. 19), such factors thereby causing deterioration of signal quality.
Hence, in order to suppress such deterioration of signal quality, it is necessary to stabilize a phase difference between the above described phase modulated signal component from the I arm side and a phase modulation signal component from the Q arm side into an appropriate value. For realizing such phase difference stabilization, it is considered to monitor the phase difference and feedback the bias voltage in the π/2 phase shifter 104.
The following patent document 2 discloses the following technique for stabilizing the above described phase difference into an appropriate value. Synchronous detection using a pilot signal from an optical signal monitored by use of either a two-photon absorption detection element or a fast linear photodiode is executed, and feedback control of a bias voltage in the π/2 phase shifter 104 is performed in such a manner that a DC component resulting from a phase deviation from π/2 is eliminated.
Patent Document 1: Pamphlet of International Publication No. 03/049333
Patent Document 2: US Patent Application Publication No. 2004/0081470